In most circumstances, the CDU-CSU and the Social Democrats (SPD) compete against one another for political office. But they've decided to make common cause in finding a successor to Joachim Gauck, who will step down as Germany's president on February 12, 2017.

The news magazine Der Spiegel is reporting that the heads of the three parties in question - Angela Merkel for the CDU, the CSU's Horst Seehofer and the SPD's Sigmar Gabriel - have agreed to field a common candidate. In separate interviews, CDU General Secretary Peter Tauber and SPD Thomas Oppermann confirmed that their parties would support a candidate without political allegiance to either side.

Although the German president is nominally the head of state, aside from certain constitutional duties, his function is largely ceremonial. But the post is high-profile and prestigious, and some past presidents have had close affiliations with political parties.

What's more, the parties' attitudes toward the new president are a good indication of what their coalition strategies could be in the far more significant general election next September, which will determine the next German chancellor.

No signal for "black and green"

Unlike the German chancellor or the US president, the German president is not chosen by popular vote, but rather selected by what's known as the Federal Convention. It's comprised in equal parts of the Bundestag parliamentary delegates and representatives from Germany's sixteen federal states. It thus reflects the relative strengths of Germany's political parties both nationwide and locally.

At present, the only possible two-party majorities would be the CDU-CSU with the SPD, which would comprise the current governing grand coalition at the national level, or the so-called "black and green option" of CDU-CSU and the Greens.

The agreement to put forward a politically neutral candidate makes the latter more unlikely, and the Greens have also signaled no interest in a trial of potentially cooperating alone with the conservatives on the issue.

"We don't want a candidate representing any particular camp in any particular direction," Green parliamentary fraction chairwoman Katrin Göring-Eckardt told DPA news agency.

Broad front against the AfD

Another option that seems to be off the table is a three-way partnership between the SPD, the Greens and the Left party, which would have a near majority in the Convention and is being touted as one likely outcome for the city government in Berlin after recent elections there.

The Left Party state premier of Thüringen, Bodo Ramelow, said that it was more important to create a broad front against the right-wing populist Alternative for Germany (AfD), after the anti-immigration and anti-EU party recorded impressive results in Berlin and the state of Mecklenburg-Western Pomerania.

"In times when the AfD is sprinting from one victory to another, we need a candidate who is acceptable to a broad spectrum," Ramelow told Spiegel. That's fairly extraordinary given the Left Party's usual hostility toward the conservatives.

But there is another reason for parties to support a non-partisan candidate for president. With the two main forces in German politics, the CDU-CSU and the SPD, recording historical lows in electoral support, it's difficult to tell who may have to negotiate with whom after the national election in September 2017. Putting forward a party neutral candidate is a way for the parties to keep their options open. But can they find a suitable candidate?

Big shoes to fill

A leading name mentioned in connection with the presidency, Federal Constitutional Court President Andreas Vosskuhle, has taken himself out of the running, citing family issues. And thus far, no one else has emerged a favorite for the job.

That may have to do with the popularity of the current president, Joachim Gauck. A former pastor and East German civil rights activist who headed the government body that dealt from 1990 to 2000 with East German secret police files, Gauck assumed the office of president in 2012, after a scandal caused his predecessor to resign.

He is well regarded by all of Germany's political parties with the exception of the Left and is enormously popular with the general populace. In a poll from May of this year, 76 percent of respondents said they were "satisfied" or "very satisfied" with Gauck as president - numbers that Germany's leading party politicians can only dream of at present.

Merkel had originally hoped that Gauck would serve a second term, but he declared himself unavailable on June 6 of this year. Now, the difficult - and revealing - search for a successor is on.